A bar soap is produced by melting and mixing raw materials such as a fatty acid soap, a polyol and an inorganic salt to give a neat soap and then solidifying the same. As the inorganic salt out of these raw materials, sodium chloride (e.g., JP-A-2002-80896), sodium sulfate (e.g., JP-A-2001-64690), a hydrogencarbonate and/or carbonate (e.g., JP-A-2001-64691) or the like is used to provide the bar soap with improved foamability and transparency.
The solidification behavior and hardness of the resulting bar soap vary depending upon the kind and proportion of such an inorganic salt. When sodium chloride is used as an inorganic salt, for example, a limitation is imposed on its proportion because, if it is added in a large proportion to provide higher hardness, the neat soap undergoes phase separation. The use of sodium sulfate involves drawbacks such as slow solidification and hence, low productivity. Further, the use of sodium carbonate results in a composition having a high melt viscosity, and thus the productivity gets worse.
There have been proposed soap compositions which use a nonionic surfactant in combination with a fatty acid soap and an inorganic salt for accelerating their solidification (JP-A-10-168494, U.S. Pat. No. 5,972,860). These soap compositions are, however, accompanied by the problem that the use of such nonionic surfactants leads to a cost increase since they are generally more expensive than fatty acid soaps.
It has also been proposed to use an insoluble soap as much as 50% or more of a sodium soap in a fatty acid soap with a view to obtaining a cleansing bar excellent in hardness and transparency (WO 96/04360). However, when an insoluble soap is added in a large proportion, sufficient foaming cannot be produced unless one or more other foaming surfactants are used in combination.